1. Field of the Invention
The present invention relates to an apparatus having a carriage, such as a recording apparatus, a reading apparatus or the like, in which a head member, such as recording means for effecting recording on a recording material or reading means for reading information held on an information holding medium such as an original, is mounted on a carriage so that the head member is reciprocally shifted along the recording material or the information holding medium.
2. Related Background Art
Among recording apparatuses having a printer function, a copying function or a facsimile function, or recording apparatuses used as output devices in composite electronic equipment or work stations including a computer or a word processor, or reading apparatuses used as input devices, there are apparatuses in which a recording head or a reading head such as a scanner is mounted on a carriage.
With this arrangement, in an apparatus of a serial type in which recording or reading is effected by main-scanning a sheet material such as a recording material or an original (information holding medium) in a direction transverse to a sheet conveying direction (sub-scanning direction), an image is formed (recording) or image information on the original is read by the recording means (recording head) or the reading means (reading head) mounted on the carriage shifted along the sheet material to complete one-line image formation or one-line image information reading and then the sheet material is fed by a predetermined amount (pitch conveyance as sub-scanning), and, by repeating such operations, an image is formed on the entire recording material or all the image information on the original is read.
In the above-mentioned apparatus of the serial type, by shifting the carriage on which the head member such as the recording head or the reading head is mounted along the sheet material in a scanning fashion and driving the head member synchronously with the scanning, the predetermined function of the head member, such as recording or reading, is effected.
For example, in order to obtain a highly fine output image by the recording apparatus of serial type, since the scanning timing of the carriage and the driving timing of the recording head must be matched with high accuracy, it is required that the scanning of the carriage be effected at a stable speed as much as possible. This is also true in the reading apparatus of a serial type.
To this end, there has been proposed a technique in which not only the function of the head member is controlled in response to a control signal for the scanning of the carriage, but also, by providing an encoder for detecting a position of the carriage during the scanning, the function of the head member is controlled synchronously with a detection timing of the encoder. However, such a technique tends to make the entire apparatus expensive and bulky.
Further, as scan driving force transmitting means for transmitting a driving force from a drive motor as a drive source to the carriage, a lead screw system and a toothed timing belt system are known. Recently, the toothed timing belt (toothed belt) has mainly been used in consideration of its low cost, ease of assembly and high accuracy. The toothed (timing) belt is suspended with predetermined tension between a driving pulley driven by a drive motor and an opposed idler pulley, and a driving force is transmitted by engagement between teeth provided on the driving pulley and teeth of the toothed belt. The carriage on which the head member is mounted is connected to the toothed belt so that it is reciprocally shifted between the pulleys as the drive motor is rotated.
For example, in the recording apparatus, as well as the recording head, ink storing means for storing ink required for image formation, means for supplying the ink to the recording head and recording head drive signal transmitting means may be provided on the carriage. Further, the carriage is supported and guided by a guide shaft or a guide rail provided substantially in parallel with the toothed belt, and the carriage is slidingly shifted on the supporting and guiding means (guide shaft or guide rail) in a condition that a load, such as the weight of the recording head, acts on the carriage. Thus, a predetermined drive transmitting force is required for the toothed belt.
In the conventional toothed belts, in order to ensure the predetermined drive transmitting force, the teeth of the toothed belt was made bulky to some extent and a height of each tooth (addendum) was increased. However, in the toothed belt having high addendum, when a tooth is engaged by a tooth of the driving pulley, vibration is generated in the toothed belt, thereby rendering the scanning speed of the carriage unstable. For this reason, in the recording apparatuses effecting highly accurate recording, carriage position detecting means such as an encoder is required, thereby preventing compactness, light weight and low cost of the apparatus.
On the other hand, when the driving force is transmitted by a toothed belt having small tooth pitch and low addendum in order to stabilize the scanning speed of the carriage, a tooth of the toothed belt floats off the driving pulley, thereby easily causing a jumping phenomenon in which idle rotation of the driving pulley is generated. In order to prevent the jumping phenomenon, it is effective to increase the tension on the toothed belt and/or to increase a diameter of the driving pulley. However, if the tension of the belt is increased, since the driving load is increased, it is required that a driving motor having large capacity be used, thereby increasing cost and making the apparatus bulky. On the other hand, if the diameter of the driving pulley is increased, similarly, the entire apparatus is made bulky. Further, since the greater the belt tension the greater the vibration caused by the engagement between the driving pulley and the belt, the effect of reduction in vibration obtained by reducing the addendum will be cancelled.